Inband tone signaling schemes using combinations of discrete frequencies have long been used in the telephone system. The primary advantage of inband tone signaling is that it shares the same spectrum that normally carries customer speech to transmit signal and control information. Sharing the voiceband is essential in situations where bandwidth is limited and dedicated control channels are either too costly or pose a degradation to service. Inband tone signaling is also well suited for short bursty types of signaling that do not occur often. One of the most common examples of inband tone signaling in the telephone system is touch tone dialing wherein inband tone signals are transmitted from a subscriber location to a central office. In other applications such as the Calling Identity Delivery on Call Waiting (CIDCW) and Analog Display Services Interface (ADSI Service), inband tone signals are transmitted from a transmitter at a central office to a detector at a subscriber location.
While reuse of the voiceband makes inband tone signaling seem like an attractive option, it also introduces significant problems related to signal recognition. Two traditional problems with inband tone signal detection are talk-off and talk-down. Talkoff occurs whenever a tone signal detector erroneously accepts signal imitations, such as those produced by speech and music, as valid tone signals. Numerous studies and experience have demonstrated that complex time varying signals such as speech and music can at times imitate some of the spectral and temporal characteristics of tone signals. These imitations are prone to triggering or talking off tone signal detectors. One of the challenges in designing tone signal detectors is making the detector resistant to signal imitations produced by speech and music, thereby mitigating the talk-off problem.
Another problem with inband tone signaling is that the tone signals must in some situations, compete with speech, music or other background noise on the line. The presence of these complex signals introduces extraneous spectral components that tend to distort and ultimately impair the detection of valid tone signals. A tone signal detector is said to have been talked down whenever it fails to recognize valid tone signals because they were masked by speech, music or noise.
Because both customer speech and tone signals share the same bandwidth and overlap in frequency, tone signal detectors for services such as CIDCW must, in the worst case, be able to simultaneously resist tone signal imitations produced by speech and music and recognize valid tone signals under very noisy circumstances.
Another constraint placed on tone signal recognition is the obvious relationship between talkoff and talkdown. In any given detector design, talkoff and talkdown are inverse. Good talkoff performance is usually achieved by sacrificing talkdown performance. Likewise, good talkdown performance is usually achieved by sacrificing talkoff performance.
It is a characteristic of tone signal detectors to employ the concept of guard action to reliably detect valid tone signals only when a certain signal-to-guard ratio is satisfied for each of the signaling frequency components in the tone signal. The signal-to-guard ratio is the ratio of the power present at a signaling frequency to the power of the voice signal in a designated guard band. The guard band is a portion of the voiceband that the detector uses to extract information about the purity of the signaling frequencies. The guard band can include the signaling spectrum. Each signaling frequency of the tone signal may have one or more guard bands. In some cases, all signaling frequencies in the tone signal may use a common guard band. If a detector requires a large positive signal-to-guard ratio, it will only validate incoming signals containing signaling frequencies that are relatively pure with respect to the guard bands. Requiring that a large signal-to-guard ratio be satisfied equates to reducing the set of possible speech and music imitations that will trigger the tone signal detector. One method of improving a detector's talkoff performance is to require that a large positive signal-to-guard ratio be satisfied for each signaling frequency before an incoming signal is recognized a valid tone signal.
Although good talkoff performance can be achieved by requiring a large positive signal-to-guard ratio, talkdown performance will most likely suffer. A valid tone signal superimposed on speech will be rejected by this detector because the signal-to-guard ratio requirement will not be satisfied. The ultimate outcome is that as tone signal detectors become more selective, they reject more signals as possible talk-offs and this in turn makes detector talk-down performance worse. From the perspective of a detector, talk-off and talk-down counterbalance one another. One cannot be made better without making the other worse.
Talk-off and talk-down performance are heavily influenced by the choice of signaling parameters employed. Generally, talk-off and talk-down are a function of:
Number of frequencies composing the tone signal PA1 Choice of signaling frequencies PA1 Temporal pattern of the tone signal PA1 Amplitude of each signaling frequency PA1 Detector's exposure time PA1 Signal-to-voice ratio for each signaling frequency PA1 Improved talk-down performance PA1 Allows talk-off performance to be improved without sacrificing talk-down performance PA1 Sensitive to and compatible with human factors issues PA1 Adaptive to various line and loop conditions PA1 Provides good signaling performance under noisy conditions PA1 Allows simple low cost detection circuitry to be used in customer premises equipment.
Values for each of these parameters are chosen to satisfy talk-off and talk-down performance criteria and also human factors considerations when customers can hear the tone signals. Based on the knowledge of typical spectral patterns of speech and music, some general recommendations can be made for each of these parameters to minimize talk-off and talk-down.
To minimize talk-off, the inband tone signal should be composed of no less than two frequency components. Studies see e.g., L. Glaser and E. Ganitta, "Speech Immunity of Push-Button Tone Signaling Systems Employ Tone Receivers with Guard Circuits," Electrical Communication, Volume 39, pages 20-243; 1964) have decisively shown that speech is less likely to imitate dual tone signals as opposed to single frequency signals. These frequencies should be chosen from the upper voiceband where the spectral power produced by speech is considerably less. The signal should be as long as possible in duration and may exhibit a pulsed temporal pattern. The tone detector should also only be exposed for the minimum necessary duration.
To minimize talk-down, the inband tone signal should contain the minimum number of frequencies required. These frequencies should also be chosen from the upper voiceband. The signal duration should be short and only single pulses should be used.
To minimize both talk-off and talk-down, the tone signal should be transmitted at the maximum allowable level.
From a human factors perspective, single frequency tone signals are preferred over multi-tone signals. Generally, the signaling frequencies should be chosen from the lower voiceband. The tone signal should be a single pulse of the minimum required duration and the amplitude of the tone signal should be sufficiently low so that it does not annoy the customer.
Table 1 below summarizes the results of the preceding discussion about choosing signaling parameters. Because some sort of balance must be reached to satisfy each of these opposing conditions, inband tone signaling parameters must be customized for the individual application.
TABLE 1 ______________________________________ Customer Parameter Talk-off Talk-down Dissatisfaction ______________________________________ Frequency Upper Upper Voiceband Lower Voiceband Voiceband and Single and Single and Multiple Frequency Frequency Frequencies Amplitude High Level High Level Low Level Temporal Long Pulse Short Pulse Short pulse Pattern ______________________________________
One particular shortcoming of conventional inband tone signaling schemes is that tone signal levels are fixed. The same tone signal level is used for all kinds of line and subscriber loop conditions. In very noisy environments, tone signals are likely to be talked down if the tone signal level is too low. On the other hand, transmitted tone signal levels must be low enough so as not to annoy customers. While higher tone signal levels compensate for noisy conditions, long loops and loud talkers, they are unacceptable for soft talkers on short loops. Fixed level signal transmission is just too inflexible to handle the wide variety of line conditions. A more dynamic signaling scheme is needed.
In view of the foregoing, it is an object of the present invention to provide an inband tone signaling method which overcomes the shortcomings of the prior art inband tone signaling techniques. More particularly, it is an object of the present invention to provide an inband tone signaling method wherein the level of each signaling frequency in the tone signal is adjusted adaptively. This permits improvements in both talk-off and talk-down performance without sacrificing important human factors considerations.